Hypergap Optical Materials.

Optical materials primarily refer to transparent insulators and semiconductors for guiding, diffracting, and nonlinearly-generating light at photon energies below the electronic bandgaps. This work proposes that a solid can be equally lossless, above the fundamental bandgap, in an energy interval dubbed the hypergap, when the conduction and valence bands are well-isolated. The optics within the hypergap could defy the conventional rules and limits set by the bandgap materials, including the low-loss negative permittivity unavailable in existing metals, the anomalous-dispersion phase matching in crystals without birefringence or microstructures, as well as the negative group-velocity dispersion across the visible spectrum unattainable in known dielectrics. High-throughput searches are performed in comprehensive material databases, predict over a hundred hypergap candidates, and experimentally verify one of them. Therefore, hypergap materials might lead to lower loss plasmonic metamaterials, easier wavelength converters in nonlinear optics, and simpler pulse stretchers or compressors in ultrafast optics, potentially transforming optics with unexplored material opportunities.